1. Field of the Invention
This invention relates to a variable displacement pump and the hydraulic controls therefor. More particularly, this invention pertains to a servo control that limits the flow rate of the pump to a value corresponding to a predetermined differential pressure across a venturi orifice. A second control regulates the pump volume to a minimum flow rate and pressurizes the hydraulic system when a valve spool is rotated. This produces a control pressure that is balanced against the pump discharge pressure and operates to regulate the volumetric capacity of the pump.
2. Description of the Prior Art
A radial piston pump is a hydraulic device in which several pistons slidable radially within the cylinders of the rotor are caused to reciprocate as the rotor turns by being held in contact with a cam or pumping ring. The ring is mounted eccentrically of the rotor axis. By varying the amount of eccentricity, the flow rate of the pump can be made to vary.
Generally, variable displacement radial piston pumps have the pumping ring mounted on bearings at opposite sides of the rotor in a so-called straddle mounted position. This method of support for the pumping ring is well known from the prior art and is shown and described in the following U.S. Pat. Nos.: 3,708,250, 3,758,899, and 4,056,042. One difficulty with the straddle mounted design involves the need for excessively large control system friction forces that result when the slide block is moved to create the eccentricity of a caged pumping ring. It is preferable to reduce the inherent frictional forces resulting from the control system so that the operating efficiency of the pump can be increased. Hydraulic controls for rotary vane and piston pumps are known in the prior art and generally provide for the pump to discharge pressure to operate a servo hydraulic control that varies the eccentricity of the pumping ring. Typical examples of this kind of control are shown in U.S. Pat. Nos. 2,462,725 and 3,796,052.